We use for sending WordPress system emails such as password recovery, new account creation, comment notifications a third party service, Sendgrid. We do not cap these emails, but in case you decide to send newsletters or other forms of mass communications, we kindly ask you to install your own SMTP gateway service.
We do not cap storage space, the single metric that we currently use for overage are page-views. The storage is optimised for static content, such as images, in the size of up to 6MB. Larger files such as videos, archives should be stored on specialised services such as Jumpshare.
As a consequence of the storage optimisation for files up to 6MB, our CDN is throttling the speed for larger files, so as not to affect normal usage. There are no limitations in terms of bandwidth used, either.
We support WordPress Multisite see Codex with subfolders only. The feature can be enabled for a new site by emailing firstname.lastname@example.org, with no additional configuration from user perspective required.